Electronic packaging is a method whereby a discrete semi-conductor device, or die, is encased to protect it from the environment and to provide easy connection of the die to an external circuit. Package materials are chosen for suitable electrical, mechanical or thermal properties. The die is then attached, wired and otherwise assembled into the package. Die such as FETs are widely used in commercial products such as microwave amplifiers, cellular telephones and satellite down-converters. High device performance, reliability and low cost packaging are desirable.
The optimum package for a particular die is determined, in part, by the desired performance characteristics of the current where the die is used. The perfect electronic package can be described as one which is transparent to the die; a package that in no way alters a signal on entering or leaving the die. Any changes to the signal should be due solely to the action of the device inside. The perfect mechanical package must be compatible to die attachment and wire bonding techniques and must have low cost manufacture and easy connection to an external circuit.
Perfect packaging does not exist due to properties inherent to the package materials. These properties include inductance, capacitance and resistance that cause signal changes and they are known as "strays" or "parasitics." At higher frequencies, exceeding 200 KHz, strays cause serious degradation in signal performance. There are several examples of strays; high frequency currents in package conductors generate magnetic fluxes that create a counter current flow. Traditional package conductor leads are made of high-nickel steels which are poor electrical conductors and dissipate signal energy, referred to as "metal losses". The geometry or arrangement of the materials in the package and how it is attached to the circuit also alter the signal. When high frequency signals flow through the several interconnections between the package and the external circuit, the abrupt changes in physical and dimensional properties cause effects commonly referred to as "radiation losses". The non-conductors of the package have capacitance properties which alter the high frequency response of the applied signal.
Perfect packages are difficult to manufacture and use. Traditional ceramic packages with refractory metal bonds have good thermal and mechanical properties but are costly to fashion and undergo time-consuming single line fabrication process. Traditional packages also have non-precise dimensions. Printed and refractory fired packages with inherent thermal shrinkage have wide tolerances and cannot therefore optimize the proximity of package interfaces. Refractory packages also have wide variations in assembly location and electrical connections to external circuits.